1. Field of the Invention
The invention relates to a semiconductor device and a process for producing the same and particularly to a technique that can be usefully applied to a semiconductor device in which a semiconductor chip is bonded onto a wiring board (an interposer) through an elastomer.
2. Prior Art
In conventional semiconductor devices (packages) such as BGA (ball grid array) and CSP (chip size package), a semiconductor chip is mounted on a wiring board called an “interposer.” The interposer functions to register the external terminal of the semiconductor chip with the portion of connection of the conductor wiring on a mounting substrate for mounting thereon the semiconductor device, such as a printed wiring board, or to perform grid conversion of the external terminal of the semiconductor chip. In the interposer, a conductor wiring having a predetermined pattern and a terminal of connection to the mounting substrate are provided on the surface of an insulating substrate.
In the semiconductor device, for example, when a tape of a polyimide, which has a coefficient of thermal expansion of about 30 ppm/° C. to 40 ppm/° C., is used as the insulating substrate in the interposer, upon the operation of the semiconductor chip to raise the temperature of the semiconductor device to the operation temperature of the semiconductor device, a difference in expansion takes place between the insulating substrate and the semiconductor chip, because the coefficient of a thermal expansion of a conventional semiconductor chip using a silicon (Si) substrate is about 2.6 ppm/° C. This causes tensile stress to be applied to the face of connection between the insulating substrate (interposer) and the semiconductor chip. Due to the application of the tensile stress, a load is applied to a portion of connection between the external terminal of the semiconductor chip and the conductor wiring, resulting in breaking of a wire or the separation of the semiconductor chip. In another case, the insulating substrate is warped, leading to the application of a load to the portion of connection between the semiconductor device and the mounting substrate and resulting in breaking of a wire. To overcome this problem, a proposal has been made on a semiconductor device wherein, for example, a semiconductor chip is mounted on the interposer through a flexible material, called an elastomer, as means for relaxing the thermal stress caused by the difference in coefficient of thermal expansion between the insulating substrate and the semiconductor chip.
An example of the semiconductor device, in which a semiconductor chip has been mounted through the elastomer, is shown in FIGS. 1 and 2. In this semiconductor device, a semiconductor chip 4 is flip chip mounted through an elastomer 3 on an interposer comprising the above type of conductor wiring 2 provided on the surface of the above type of insulating substrate 1, and the conductor wiring 2 in its portion protruded in an opening 1A of the insulating substrate 1 and an opening 3A of the elastomer 3 is deformed to connect the conductor wiring 2 in its protruded portion to an external terminal 401 in the semiconductor chip 4. Here FIG. 1 is a typical plan view of the BGA-type semiconductor device, and FIG. 2 a typical cross-sectional view taken on line G-G′ of FIG. 1.
In the BGA-type semiconductor device shown in FIGS. 1 and 2, the elastomer 3 and the conductor wiring 2 in its deformed portion absorb the thermal stress caused by the difference in coefficient of thermal expansion between the semiconductor chip 4 and the insulating substrate 1 (interposer) and thus can relax the thermal stress. Further, as shown in FIG. 2, a via hole 1B is provided in the insulating substrate 1, and a ball terminal 6 for connection to the conductor wiring 2 is provided in the via hole 1B portion. The ball terminal 6 is used, for example, in mounting the semiconductor device on a mounting substrate such as a mother board, as a terminal of connection between the wiring conductor 2 and wiring (terminal) on the mounting substrate.
A production process of the BGA-type semiconductor device shown in FIGS. 1 and 2 will be briefly explained. At the outset, as shown in FIG. 3A, for example, an interposer (a wiring board) comprising a conductor wiring 2 having a predetermined pattern provided on the surface of the insulating substrate 1 provided with an opening 1A for bonding and a via hole 1B at respective predetermined positions is provided. In this case, as shown in FIGS. 1 and 3A, the conductor wiring 2 is formed so that a part of the conductor wiring 2 is projected into the opening 1A for bonding while another part of the conductor wiring 2 covers the via hole 1B.
The interposer is produced, for example, by forming the opening 1A for bonding and the via hole 1B using a mold in the insulating substrate 1 such as a polyimide tape, then forming a thin conductor layer formed of a copper foil or the like on the surface of the insulating substrate 1, and patterning the thin conductor layer by etching or the like to form the conductor wiring 2. Another example of the method for producing the interposer comprises the steps of forming the thin conductor layer on the surface of the insulating substrate 1, then forming the opening 1A for bonding and the via hole 1B in the insulating substrate 1 by laser etching using a carbonic gas laser, an excimer laser or the like, and patterning the thin conductor layer by etching or the like to form the conductor wiring 2.
In this case, the insulating substrate 1 is generally in a tape form which is continuous in one direction, and, in many cases, a large number of semiconductor devices are continuously produced in a single insulating substrate 1 of the above type by a reel to reel method, followed by taking-off of predetermined regions (package regions) from the insulating substrate 1 to prepare individual pieces. The region as shown in FIG. 3A is repeatedly formed over the whole insulating substrate 1.
Next, in the step of elastomer bonding, as shown in FIG. 3B, an elastomer 3 having an opening provided at a position corresponding to the opening 1A for bonding in the insulating substrate 1 is bonded to the surface of the interposer, in other words, the interposer in its surface on which the conductor wiring 2 has been formed. For example, a three-layer structure comprising an elastic material having a coefficient of thermal expansion of not more than 100 ppm/° C. or a modulus of elasticity of not more than 1000 MPa and an adhesive layer provided on both sides of the elastic material may be used as the elastomer. The elastic material is preferably a porous material highly permeable to water. The adhesive layer is formed of, for example, a heat-curable resin which has been cured to a stage B.
Next, in the step of bonding a semiconductor chip, as shown in FIG. 3C, the semiconductor chip 4 is bonded onto the elastomer 3. At that time, the semiconductor chip 4 is registered so that the external terminal 401 is located within the opening 3A in the elastomer 3 and the external terminal 401 overlaps with the conductor wiring 2 in a planner manner, followed by bonding onto the elastomer 3. Thereafter, heating is carried out to fully cure the adhesive layer in the elastomer 3.
Next, the conductor wiring 2 in its portion projected into the opening 1A for bonding in the insulating substrate 1 is press cut with a bonding tool in the step of wire connection, and, as shown in FIG. 3D, the cut portion of the conductor wiring 2 is pushed into the opening 3A in the elastomer 3 and is deformed. Thereafter, for example, ultrasonic vibration is applied from the bonding tool to the conductor wiring 2 to connect the conductor wiring 2 to the semiconductor chip in its external terminal 401. In this case, the conductor wiring 2 in its portion projected into the opening 1A for bonding is partly narrowed in its predetermined position so that, upon press cutting with the bonding tool, the projection portion can be connected to a predetermined external terminal, although this is not shown in the drawing.
Next, in the step of sealing, an insulator 5 formed of, for example, a heat-curable epoxy resin is poured through the opening 1A for bonding in the insulating substrate 1 and is cured to seal the portion of connection between the conductor wiring 2 and the semiconductor chip in its external terminal 401.
Thereafter, in the step of connection of a ball terminal, a ball terminal 6 formed of, for example, a Pb·Sn-base solder is connected to the via hole 1B in the insulating substrate 1, followed by cutting of the insulating substrate 1 (interposer) to take off predetermined regions (package regions) to prepare individual pieces. Thus, the BGA-type semiconductor device as shown in FIGS. 1 and 2 can be prepared.
Further, in the semiconductor device shown in FIGS. 1 and 2, for example, a center pad-type semiconductor chip, wherein the external terminal 401 is provided around the center line of the surface of a silicon substrate provided with a circuit such as DRAM (a dynamic random access memory), is used as the semiconductor chip 4. Another example of the semiconductor device is a semiconductor device using a peripheral pad-type semiconductor chip wherein the external terminal 401 is provided around the end in the long side direction or the short side direction of the surface of the silicon substrate provided with a circuit. The connection terminal mounted on the mounting substrate is not limited to the ball terminal 6, and, for example, a connection terminal may be used wherein a flat connection terminal (a land) is formed using a copper double clad laminate board on the face of connection to the mounting substrate.
In the case of the semiconductor device as shown in FIGS. 1 and 2, the portion of connection between the conductor wiring 2 and the semiconductor chip in its external terminal 401 is merely sealed with the insulator 5. Therefore, the semiconductor chip 4 is externally exposed. For example, in the case of MCM (multi-chip module), the semiconductor device is used as a component of an electronic device which, in the state of being mounted on a mounting substrate such as a mother board, has one function. In this case, when the semiconductor chip 4 is externally exposed, for example, at the time of mounting of the semiconductor device on the mounting substrate or at the time of use of the semiconductor substrate mounted on the mounting substrate, a problem occurs such that the exposed surface of the semiconductor chip 4 is damaged, or the corner portion of the semiconductor chip 4 is broken.
Further, since the semiconductor chip 4 and the elastomer 3 are in the exposed state, water is likely to penetrate through the adhesive interface of the semiconductor chip 4 and the elastomer 3. When a porous material is used as the elastic material used in the elastomer 3, the elastomer 3 is likely to absorb water. This poses a problem that the absorbed or penetrated water causes the separation of the semiconductor chip 4, or the conductor wiring 2, the internal wiring in the semiconductor chip 4 or the like is likely to be attacked resulting in deteriorated electrical characteristics.
To overcome this problem, a semiconductor device, wherein not only the connection between the conductor wiring 2 and the semiconductor chip in its external terminal 401 but also, as shown in FIG. 4, the periphery of the semiconductor chip 4 and the elastomer 3 has been sealed with the insulator 5, has been proposed and used.
The semiconductor device shown in FIG. 4 is produced as follows. In the procedure as shown in FIGS. 3A, 3B, 3C, and 3D, the semiconductor chip 4 is bonded onto the interposer through the elastomer 3, and the conductor wiring 2 is connected to the semiconductor chip in its external terminal 401. Thereafter, in the step of sealing, the periphery of the semiconductor chip 4 and the elastomer 3 and the connection between the conductor wiring 2 and the semiconductor chip in its external terminal 401 are sealed with the insulator 5, for example, by transfer molding using a mold. The ball terminal 6 is then connected, and the interposer in its predetermined regions is taken off to prepare individual pieces.
In the step of sealing, when the periphery of the semiconductor chip 4 and the elastomer 3 is sealed by transfer molding, for example, as shown in FIG. 5A, the interposer, on which the semiconductor chip 4 has been flip chip mounted, is sandwiched and fixed between an upper die 7, provided with a cavity 702 for receiving the semiconductor chip 4 and the elastomer 3, and a lower die 8 in a flat plate form. In this case, for example, between the upper die 7 and the lower die 8, as shown in FIG. 5A, in addition to the cavity 702, provided are spaces, for example, a pot 704, into which the insulator 5 for sealing the semiconductor chip 4 is introduced, a gate 701 for pouring the insulator 5, which has been introduced into the pot 704 and melted, into the cavity 702, and an air vent 703 which, when the insulator 5 has been poured through the gate 701, functions to release air within the cavity 702 to the outside of the assembly.
In the case of the transfer molding, after the heat-curable resin as the insulator 5 is introduced into the pot 704 and melted, as shown in FIG. 5B, the melted insulator 5 is pressed by means of a plunger 10. This permits the insulator 5 to be passed through the gate 701 and to be poured into the cavity 702. After the insulator 5 is poured into the cavity 702 to fill the periphery of the semiconductor chip 4 and the elastomer 3 with the insulator 5, the insulator 5 is cured, followed by the removal of the upper die 7 and the lower die 8. Thus, the periphery of the semiconductor chip 4 and the elastomer 3 and the connection between the conductor wiring 2 and the semiconductor chip in its external terminal 401 are sealed with the insulator 5.
Methods for sealing the periphery of the semiconductor chip 4 and the elastomer 3 with the insulator 5 include, in addition to the above transfer molding using a mold, a method wherein the whole surface of the interposer, on which the semiconductor chip 4 has been flip chip mounted, is coated with an insulator 5 formed of a heat-curable resin or the like.
In the above prior art method, however, in the step of sealing, when the periphery of the semiconductor chip 4 is sealed with the insulator 5 by the transfer molding using a mold, the periphery of the elastomer 3 is also sealed with the insulator 5.
In general, a porous material, which is highly flexible and highly permeable to water, is in many cases used as the elastomer 3 and, thus, water is likely to be incorporated into the pore portion present in the material. The water incorporated into the elastomer 3 is vaporized and expanded, for example, in the step of heating for mounting the semiconductor device on the mounting substrate. At that time, when the periphery of the elastomer 3 is sealed with the insulator as in the case of the semiconductor device shown in FIG. 4, however, the vaporized water cannot be released to the outside of the semiconductor device. This poses a problem that thermal shock caused by the vaporization and expansion of the water within the elastomer 3 is likely to cause the separation of the semiconductor chip 4 or the interposer.
Further, when the water incorporated into the elastomer 3 cannot be released to the outside of the semiconductor device, metal portions such as the conductor wiring 2, the internal wiring in the semiconductor chip 4 and the like are likely to be attacked by the incorporated water and, thus, disadvantageously, the electrical characteristics of the semiconductor device are likely to be deteriorated.